1. Field of the Invention
The present invention is directed generally to electrical power supplies, and more particularly, to a system for regulating a power supply which includes circuitry to permit equalizing current between a plurality of power supply modules which are coupled together to allow each module to supply substantially the same current to a power distribution bus.
2. Description of the Prior Art
It is known in the prior art that it is often advantageous to provide a plurality of power supply modules to drive a particular load rather than to design and construct a single power supply for that purpose. A module comprises a unitary power supply of a given configuration. These advantages come from several sources, such as being able to utilize readily available components rather than requiring power components that may be unduly expensive or unavailable in the present state of the art. There is an advantage in being able to design and construct standardized individual power supply units that can be selected and utilized in plurality for driving a particular load under consideration. Also, a margin of safety can be designed into such a system by providing more power supply modules than would normally be required in order to accommodate failures in individual supplies.
It has been common in the prior art systems that involve the use of multiple power supply modules to drive a given load to operate the modules in a current limit mode. This mode of operation results in the majority of the individual power supply modules being operated at the maximum power output capacity, with only one module taking up the balance of the power required for the particular load. Consider, for example, where a particular load has six modules coupled to it and the output capacity of four of the modules operating at maximum output is not sufficient to drive the load. Four of the modules would be operated at the maximum capacity with the balance of the load supplied by the fifth module. In such a configuration, the sixth module supply would not be operative and would be idle. In such a system, if the load were variable and would increase beyond the capacity of the fifth power supply module when added to the four modules operating at maximum capacity, the sixth module would then be brought into operation to supply the balance of the load. It is apparent that the system must be designed such that the maximum load that can be encountered may be supplied by the number of modules available.
It is clear, however, that a power supply of this configuration results in unequal stress on the power supply modules, since some of the modules are operating at maximum capacity at all times, some of the modules are operative at varying capacities depending upon load requirements, and some of the modules may be inoperative for long periods of time. The stress resulting from such operation tends to result in a higher failure rate for those modules that are operated at maximum capacity for the greatest length of time.
It has been found that parallel coupled power supply modules with output impedances known to be substantially equal can be made to share common loads by maintaining the amplitude of the generated voltages substantially equal. Because it is impossible to guarantee that the output voltage for two or more conventional modules will be exactly the same, very poor current paralleling characteristics are obtained in the prior art. As an example, FIG. 1 shows an ideal conventional power supply, which appears as a constant voltage source for output currents less than a limit point at which the output current is limited to a maximum level to prevent damage to the power supply in the even of a shorted output, and which appears as a constant current source for overload conditions as shown in FIG. 2X. Output voltage and current characteristics are shown for three conventional power supply modules in FIG. 3. A first power supply module, denoted by PSM1, has an output voltage of 5.050 volts and a current capacity of 40 amp. A second power supply module PSM2 has an output voltage of 5.025 volts and a current capacity of 35 amp. The third supply module, PSM3, has an output voltage of 5.000 and a maximum current capacity of 42 amp. If the three modules are parallelled, module PSM1 will provide the entire load current due to its higher available supply voltage until the load current exceeds the total current limit threshold of 40 amp. When module PSM1 enters the current limit, the supply voltage drops out of regulation and module PSM2 picks up the additional load current until the load is increased to the point where module PSM2 enters current limit. Module PSM3 then picks up the additional current requirement until the maximum output current is reached. A corresponding graph of output voltage and current for the three modules operating in parallel is shown in FIG. 4.
Prior art modular power systems are designed to operate in this manner. However, it is a disadvantage that one power supply module must go into the overload current limiting mode before a second module starts to contribute power to the load. The module that produces the highest output voltage will run the hottest and thus may be expected to fail first. The dynamic response of the system also suffers because one supply must regulate the output voltage for the other supplies that are in the current limit. Further, prior art systems require complex interconnections for sensing the current supplied by each individual module and controlling the supplies to equalize the currents applied to the load.
The present invention has been designed to permit parallel operation without requiring any additional coupling of control signals between modules for current balancing. The invention allows power supply modules to operate in a voltage regulation mode and contribute substantially equal portions of current to a common output load. Current sharing is accomplished even though the output voltage regulation characteristics are not exactly matched between the various power supply modules.